Sen. Mark R. Warner announced Tuesday that he would not make another run for Virginia governor, essentially setting the field for an unusually colorful gubernatorial race next year.

The Democratic senator’s decision boosts the possibility of an intensely partisan matchup. Barring a surprise entrant, the Democratic field appears clear for outspoken businessman Terry McAuliffe, who once headed the national Democratic Party. On the Republican side, Lt. Gov. Bill Bolling and Attorney General Ken Cuccinelli II are battling for their party’s nod. Cuccinelli is a tea party favorite with strongly held conservative views.

Warner, who was the commonwealth’s chief executive from 2002 through the start of 2006, had refused to rule out a return engagement in recent weeks, and a series of polls confirmed that he remains popular statewide and would be the instant favorite if he ran. But few Virginia Democrats expected him to run, and Warner said Tuesday that he had decided to stay put after giving the question “serious, heartfelt consideration.”

“I loved being governor, but I have a different job now — and it’s here, in the United States Senate,” Warner said in a statement issued by his office. “I hope my value add in Congress is to continue working hard every day to not simply blame the other side, but to actually try to find common ground so we can get stuff done.”

Democrats can take heart that they scored clear victories in the election this month, with President Obama winning Virginia a second time and Timothy M. Kaine (D) prevailing in a high-profile Senate race. With a growing minority population and the continuing leftward trend of the Northern Virginia suburbs, both parties agree that the state is firmly in the purple column.

But Democrats in the commonwealth do not always do as well turning out voters in non-federal election years, and since 1977 Virginia has consistently elected governors from the opposite party of the previous year’s presidential winner.

In Virginia, governors can serve multiple terms, as long as they are not consecutive.

Popular Gov. Robert F. McDonnell (R) is publicly backing Bolling for the Republican nomination, while a significant portion of the conservative base supports Cuccinelli. Democrats view Cuccinelli as the weaker general election opponent because of his views on abortion and other social issues, but Bolling faces a hurdle because the nomination will be decided not in a primary but at a state party convention, which tends to favor conservatives.

McAuliffe — a New York native and former Democratic National Committee chairman with close ties to the Clintons — will likely be attacked by Republicans as a liberal and a carpetbagger with weak ties to the state.

After losing the Democratic gubernatorial primary to state Sen. R. Creigh Deeds (Bath) in 2009, McAuliffe spent three years making clear that he planned to run again, working to strengthen his political ties across the state while building an electric car company.

McAuliffe publicly declared early this month that he was entering the race, and he has hired two key aides. Robby Mook, the former head of the Democratic Congressional Campaign Committee, will serve as campaign manager, and Levar Stoney, former executive director of the state party, will be deputy campaign manager. McAuliffe also has moved to install an ally, Del. Charniele Herring (Alexandria), as head of the state Democratic Party after Brian Moran stepped down from the post.

With Warner’s popularity still sky-high — a Quinnipiac poll released last week gave him the highest job approval rating, 58 percent, of any statewide elected official — McAuliffe will be eager to have the former governor’s help on the campaign trail.

“One of the reasons I am running for governor is to continue the Warner tradition of common sense leadership that puts economic growth and fiscal responsibility ahead of divisive partisan issues,” McAuliffe said on his Facebook page Tuesday.

Warner, for his part, has long made clear that he loved being governor and missed the post. “Believe me, being governor was the best job I ever had,” he said in his statement.

He also has publicly acknowledged being frustrated at times with the Senate, where the pace is slower and bipartisan compromise has been hard to come by.

Yet if Warner had won the governorship, his Senate seat would be vulnerable to Republicans in the next election. And the atmosphere in Richmond is different than it was when Warner occupied the governor’s mansion.

“Warner’s success as governor was heavily dependent on a cadre of moderate Republicans in both chambers of the state legislature that worked with him,” said Stephen Farnsworth, a University of Mary Washington political science professor.

Now, Farnsworth said, many of those moderates are gone, so “the idea that a Democratic governor could find 20 Republicans in Richmond willing to make a deal the way that Warner did, that’s not today’s Richmond.”

Instead, Warner, who will be up for reelection in 2014, will focus on efforts to strike a major deficit-reduction deal in the Senate. Although he has not been allowed a seat at the main negotiating table by Democratic leaders, Warner and Sen. Saxby Chambliss (R-Ga.) have spearheaded a bipartisan “Gang of Six” that has sought to influence the process by coming up with its own budget agreement.